1. Field of the Invention
The present invention relates to nontoxic gas generating compositions are described that upon combustion, rapidly generate gases that are useful for inflating occupant safety restraints in motor vehicles and specifically, the invention relates to thermally stable nonazide gas generants having not only acceptable burn rates, but that also, upon combustion, exhibit a relatively high gas volume to solid particulate ratio at acceptable flame temperatures.
The evolution from azide-based gas generants to nonazide gas generants is well-documented in the prior art. The advantages of nonazide gas generant compositions in comparison with azide gas generants have been extensively described in the patent literature, for example, U.S. Pat. Nos. 4,370,181; 4,909,549; 4,948,439; 5,084,118; 5,139,588 and 5,035,757, the discussions of which are hereby incorporated by reference.
In addition to a fuel constituent, pyrotechnic nonazide gas generants contain ingredients such as oxidizers to provide the required oxygen for rapid combustion and reduce the quantity of toxic gases generated, a catalyst to promote the conversion of toxic oxides of carbon and nitrogen to innocuous gases, and a slag forming constituent to cause the solid and liquid products formed during and immediately after combustion to agglomerate into filterable clinker-like particulates. Other optional additives, such as burning rate enhancers or ballistic modifiers and ignition aids, are used to control the ignitability and combustion properties of the gas generant.
One of the disadvantages of known nonazide gas generant compositions is the amount and physical nature of the solid residues formed during combustion. The solids produced as a result of combustion must be filtered and otherwise kept away from contact with the occupants of the vehicle. It is therefore highly desirable to develop compositions that produce a minimum of solid particulates while still providing adequate quantities of a nontoxic gas to inflate the safety device at a high rate.
The use of phase stabilized ammonium nitrate is desirable because it generates abundant nontoxic gases and minimal solids upon combustion. To be useful, however, gas generants for automotive applications must be thermally stable when aged for 400 hours or more at 107.degree. C. The compositions must also retain structural integrity when cycled between -40.degree. C. and 107.degree. C.
Often, gas generant compositions incorporating phase stabilized or pure ammonium nitrate exhibit poor thermal stability, and produce unacceptably high levels of toxic gases, CO and NO.sub.x for example, depending on the composition of the associated additives such as plasticizers and binders. In addition, ammonium nitrate contributes to poor ignitability, lower burn rates, and performance variability. Several known gas generant compositions incorporating ammonium nitrate utilize well known ignition aids such as BKNO.sub.3 to solve this problem. However, the addition of an ignition aid such as BKNO.sub.3 is undesirable because it is a highly sensitive and energetic compound, and furthermore, contributes to thermal instability and an increase in the amount of solids produced.
Certain gas generant compositions comprised of ammonium nitrate are thermally stable, but have burn rates less than desirable for use in gas inflators. To be useful for passenger restraint inflator applications, gas generant compositions generally require a burn rate of at least 0.4 inch/second (ips) or more at 1000 psi. Gas generants with burn rates of less than 0.40 ips at 1000 psi do not ignite reliably and often result in "no-fires" in the inflator.
Yet another problem that must be addressed is that the U.S. Department of Transportation (DOT) regulations require "cap testing" for gas generants. Because of the sensitivity to detonation of fuels often used in conjunction with ammonium nitrate, most propellants incorporating ammonium nitrate do not pass the cap test unless shaped into large disks, which in turn reduces design flexibility of the inflator.
Accordingly, many nonazide propellants based on ammonium nitrate cannot meet requirements for automotive applications.
2. Description of the Related Art
A description of the related art follows, the complete teachings of which are herein incorporated by reference.
U.S. Pat. No. 5,545,272 to Poole discloses the use of gas generant compositions consisting of nitroguanidine (NQ), at a weight percent of 35%-55%, and phase stabilized ammonium nitrate (PSAN) at a weight percent of 45%-65%. NQ, as a fuel, is preferred because it generates abundant gases and yet consists of very little carbon or oxygen, both of which contribute to higher levels of CO and NOx in the combustion gases. According to Poole, the use of phase stabilized ammonium nitrate (PSAN) or pure ammonium nitrate is problematic because many gas generant compositions containing the oxidizer are thermally unstable. Poole has found that combining NQ and PSAN in the percentages given results in thermally stable gas generant compositions. However, Poole reports burn rates of only 0.32-0.34 inch per second, at 1000 psi. As is well known, burn rates below 0.4 inch per second at 1000 psi are simply too low for confident use within an inflator.
In U.S. Pat. No. 5,531,941 to Poole, Poole teaches the use of PSAN, and two or more fuels selected from a specified group of nonazide fuels. Poole adds that gas generants using ammonium nitrate (AN) as the oxidizer are generally very slow burning with burning rates at 1000 psi typically less than 0.1 inch per second. He further teaches that for air bag applications, burning rates of less than about 0.4 to 0.5 inch per second are difficult to use. The use of PSAN is taught as desirable because of its propensity to produce abundant gases and minimal solids, with minimal noxious gases. Nevertheless, Poole recognizes the problem of low burn rates and thus combines PSAN with a fuel component containing a majority of TAGN, and if desired one or more additional fuels. The addition of TAGN increases the burn rate of ammonium nitrate mixtures. According to Poole, TAGN/PSAN compositions exhibit acceptable burn rates of 0.59-0.83 inch/per second. TAGN, however, is a sensitive explosive that poses safety concerns in processing and handling. In addition, TAGN is classified as "forbidden" by the Department of Transportation, therefore complicating raw material requirements.
In U.S. Pat. No. 5,500,059 to Lund et al., Lund states that burn rates in excess of 0.5 inch per second (ips) at 1,000 psi, and preferably in the range of from about 1.0 ips to about 1.2 ips at 1,000 psi, are generally desired. Lund discloses gas generant compositions comprised of a 5aminotetrazole fuel and a metallic oxidizer component. The use of a metallic oxidizer reduces the amount of gas liberated per gram of gas generant, however, and increases the amount of solids generated upon combustion.
The gas generant compositions described in Poole et al, U.S. Pat. Nos. 4,909,549 and 4,948,439, use tetrazole or triazole compounds in combination with metal oxides and oxidizer compounds (alkali metal, alkaline earth metal, and pure ammonium nitrates or perchlorates) resulting in a relatively unstable generant that decomposes at low temperatures. Significant toxic emissions and particulate are formed upon combustion. Both patents teach the use of BKNO.sub.3 as an ignition aid.
The gas generant compositions described in Poole, U.S. Pat. No. 5,035,757, result in more easily filterable solid products but the gas yield is unsatisfactory.
Chang et al, U.S. Pat. No. 3,954,528, describes the use of TAGN and a synthetic polymeric binder in combination with an oxidizing material. The oxidizing materials include pure AN although, the use of PSAN is not suggested. The patent teaches the preparation of propellants for use in guns or other devices where large amounts of carbon monoxide, nitrogen oxides, and hydrogen are acceptable and desirable. Because of the practical applications involved, thermal stability is not considered a critical parameter.
Grubaugh, U.S. Pat. No. 3,044,123, describes a method of preparing solid propellant pellets containing AN as the major component. The method requires use of an oxidizable organic binder (such as cellulose acetate, PVC, PVA, acrylonitrile and styrene-acrylonitrile), followed by compression molding the mixture to produce pellets and by heat treating the pellets. These pellets would certainly be damaged by temperature cycling because commercial ammonium nitrate is used, and the composition claimed would produce large amounts of carbon monoxide.
Becuwe, U.S. Pat. No. 5,034,072, is based on the use of 5-oxo-3-nitro-1,2,4-triazole as a replacement for other explosive materials (HMX, RDX, TATB, etc.) in propellants and gun powders. This compound is also called 3-nitro-1,2,4-triazole-5-one ("NTO"). The claims appear to cover a gun powder composition which includes NTO, AN and an inert binder, where the composition is less hygroscopic than a propellant containing ammonium nitrate. Although called inert, the binder would enter into the combustion reaction and produce carbon monoxide making it unsuitable for air bag inflation.
Lund et al, U.S. Pat. No. 5,197,758, describes gas generating compositions comprising a nonazide fuel which is a transition metal complex of an aminoarazole, and in particular are copper and zinc complexes of 5-aminotetrazole and 3-amino-1,2,4-triazole which are useful for inflating air bags in automotive restraint systems, but generate excess solids.
Wardle et al, U.S. Pat. No. 4,931,112, describes an automotive air bag gas generant formulation consisting essentially of NTO (5-nitro-1,2,4-triazole-3-one) and an oxidizer wherein said formulation is anhydrous.
Ramnarace, U.S. Pat. No. 4,111,728, describes gas generators for inflating life rafts and similar devices or that are useful as rocket propellants comprising ammonium nitrate, a polyester type binder and a fuel selected from oxamide and guanidine nitrate. Ramnarace teaches that ammonium nitrate contributes to burn rates lower than those of other oxidizers and further adds that ammonium nitrate compositions are hygroscopic and difficult to ignite, particularly if small amounts of moisture have been absorbed.
Bucerius et al, U.S. Pat. No. 5,198,046, teaches the use of diguanidinium-5,5'-azotetrazolate (GZT) with KNO.sub.3 as an oxidizer, for use in generating environmentally friendly, non-toxic gases. Bucerius teaches away from combining GZT with any chemically unstable and/or hygroscopic oxidizer. The use of other amine salts of tetrazole such as bis-(triaminoguanidinium)-5,5'-azotetrazolate (TAGZT) or aminoguanidinium-5,5'-azotetrazolate are taught as being much less thermally stable when compared to GZT.
Boyars, U.S. Pat. No. 4,124,368, describes a method for preventing detonation of ammonium nitrate by using potassium nitrate.
Mishra, U.S. Pat. No. 4,552,736, and Mehrotra et al, U.S. Pat. No. 5,098,683, describe the use of potassium fluoride to eliminate expansion and contraction of ammonium nitrate in transition phase.
Chi, U.S. Pat. No. 5,074,938, describes the use of phase stabilized ammonium nitrate as an oxidizer in propellants containing boron and as useful in rocket motors.
In U.S. Patent 5,125,684 to Cartwright, an extrudable propellant for use in crash bags is described as comprising an oxidizer salt, a cellulose-based binder and a gas generating component. Cartwright also teaches the use of "at least one energetic component selected from nitroguanidine (NG), triaminoguanidine nitrate, ethylene dinitramine, cyclotrimethylenetrinitramine (RDX), cyclotetramethylenetetranitramine (HMX), trinitrotoluene (TNT), and pentaerythritol tetranitrate (PETN) . . . ."
In U.S. Pat. No. 4,925,503 to Canterbury et al, an explosive composition is described as comprising a high energy material, e.g., ammonium nitrate and a polyurethane polyacetal elastomer binder, the latter component being the focus of the invention. Canterbury also teaches the use of a "high energy material useful in the present invention . . . preferably one of the following high energy materials: RDX, NTO, TNT, HMX, TAGN, nitroguanidine, or ammonium nitrate . . . ."
Hass, U.S. Pat. No. 3,071,617, describes long known considerations as to oxygen balance and exhaust gases.
Stinecipher et al, U.S. Pat. No. 4,300,962, describes explosives comprising ammonium nitrate and an ammonium salt of a nitroazole.
Prior, U.S. Pat. No. 3,719,604, describes gas generating compositions comprising aminoguanidine salts of azotetrazole or of ditetrazole.
Poole, U.S. Pat. No. 5,139,588, describes nonazide gas generants useful in automotive restraint devices comprising a fuel, an oxidizer and additives.
Hendrickson, U.S. Pat. No. 4,798,637, teaches the use of bitetrazole compounds, such as diammonium salts of bitetrazole, to lower the burn rate of gas generant compositions. Hendrickson describes burn rates below 0.40 ips, and an 8% decrease in the burn rate when diammonium bitetrazole is used.
Chang et al, U.S. Pat. No. 3,909,322, teaches the use of nitroaminotetrazole salts with oxidizers such as pure ammonium nitrate, HMX, and 5-ATN. These compositions are used as gun propellants and gas generants for use in gas pressure actuated mechanical devices such as engines, electric generators, motors, turbines, pneumatic tools, and rockets. In contrast to the amine salts disclosed by Hendrickson, Chang teaches that gas generants comprised of 5-aminotetrazole nitrate and salts of nitroaminotetrazole exhibit burn rates in excess of 0.40 ips. On the other hand, Chang also teaches that gas generants comprised of HMX and salts of nitroaminotetrazole exhibit burn rates of 0.243 ips to 0.360 ips. No data is given with regard to burn rates associated with pure AN and salts of nitroaminotetrazole.
Highsmith et al, U.S. Pat. No. 5,516,377, teaches the use of a salt of 5-nitraminotetrazole, NQ, a conventional ignition aid such as BKNO.sub.3, and pure ammonium nitrate as an oxidizer, but does not teach the use of phase stabilized ammonium nitrate. Highsmith states that a composition comprised of ammonium nitraminotetrazole and strontium nitrate exhibits a burn rate of 0.313 ips. This is to low for automotive application. As such, Highsmith emphasizes the use of metallic salts of nitraminotetrazole.
Onishi et al, U.S. Pat. No. 5,439,251, teaches the use of a tetrazole amine salt as an air bag gas generating agent comprising a cationic amine and an anionic tetrazolyl group having either an alkyl with carbon number 1-3, chlorine, hydroxyl, carboxyl, methoxy, aceto, nitro, or another tetrazolyl group substituted via diazo or triazo groups at the 5-position of the tetrazole ring. The inventive thrust is to improve the physical properties of tetrazoles with regard to impact and friction sensitivity, and therefore does not teach the combination of an amine or nonmetal tetrazole salt with any other chemical.
Lund et al, U.S. Pat. No. 5,501,823, teaches the use of nonazide anhydrous tetrazoles, derivatives, salts, complexes, and mixtures thereof, for use in air bag inflators. The use of bitetrazole-amines, not amine salts of bitetrazoles, is also taught.
Based on the above, the need remains for a PSAN-based gas generant which is thermally stable at 107C, ignites readily and without delay, has a burn rate at 1000 psi of greater than 0.40-0.50 ips, and contains no sensitive explosive compounds.